System and method for controlling the environment around one or more vessels in a glass manufacturing system

ABSTRACT

A system and method are described herein that control the environment (e.g., oxygen, hydrogen, humidity, temperature, gas flow rate, pressure) around one or more vessels in a glass manufacturing system. In the preferred embodiment, the system includes a closed-loop control system and a capsule that are used to control the level of hydrogen around the exterior (non glass contact surface) of the vessel(s) so as to suppress the formation of gaseous inclusions and surface blisters in glass sheets. In addition, the closed-loop control system and capsule can be used to help cool molten glass while the molten glass travels from one vessel to another vessel in the glass manufacturing system. Moreover, the closed-loop control system and capsule can be used to maintain an atmosphere with minimal oxygen around the vessel(s) so as to reduce the oxidation of precious metals on the vessel(s).

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. patent application Ser. No. 11/116,669,filed on Apr. 27, 2005 now abandoned and entitled “SYSTEM AND METHOD FORCONTROLLING THE ENVIRONMENT AROUND ONE OR MORE VESSELS IN A GLASSMANUFACTURING SYSTEM,” the content of which is relied upon andincorporated hereby in its entirety, and the benefit of priority under35 U.S.C. § 120 is hereby claimed.

TECHNICAL FIELD

The present invention relates to a system and method for controlling theenvironment (e.g., oxygen, hydrogen, humidity, temperature, gas flowrate) around one or more vessels in a glass manufacturing system.

BACKGROUND

Flat panel display devices like Liquid Crystal Displays (LCDs) utilizeflat glass sheets. A preferred technique for manufacturing these glasssheets is the fusion process. In the fusion process, the glass sheetsare made by using vessels that contain refractory/precious metals, e.g.platinum or platinum alloys. The precious metals are generallyconsidered to be inert in relation to most glasses, and thus should notcause any inclusions in the glass sheets.

However, this is not necessarily valid. There are oxidation reactionsthat occur at the metal/glass interface inside the vessels which lead tothe generation of gaseous inclusions in the glass melt and thus theglass sheet. One of the more common oxidation reactions that occurs atthe metal/glass interface is the conversion of negatively charged oxygenions to molecular oxygen which is caused by the thermal breakdown ofwater and hydroxyl species in the glass melt. This phenomenon occursbecause at the elevated temperatures of glass melting and delivery, alow partial pressure of hydrogen exists in the glass melt. And, whenhydrogen comes in contact with the refractory/precious metal vesselcontaining the glass melt, the hydrogen rapidly permeates out of thevessel, depleting the metal/glass interface of hydrogen. Based on thechemical balance, for every mole of hydrogen that leaves the vessel, ½mole of oxygen is left behind at the glass/metal interface. Thus, ashydrogen leaves the vessel, the oxygen level or partial pressure ofoxygen at the metal/glass interface increases, which leads to thegeneration of blisters or gaseous inclusions in the glass melt. Inaddition, there are other reactions which involve the catalyzing oroxidation of other species in the glass melt such as halogens (Cl, F,Br) which can lead to the generation of gaseous inclusions. Further, theoxidation reactions can occur due to electrochemical reactions at themetal/glass interface. These electrochemical reactions can be associatedwith thermal cells, galvanic cells, high AC or DC current applicationsand grounding situations.

Today, there are several known methods that can be used to address theseproblematical oxidation reactions which cause the formation of gaseousinclusions in the glass sheet. One known method that can be used to helpminimize the formation of gaseous inclusions in glass sheets involvesthe use of arsenic as a fining agent within the fusion process. Arsenicis among the highest temperature fining agents known, and, when added tothe molten glass bath, it allows for O₂ release from the glass melt athigh melting temperatures (e.g., above 1450° C.). This high temperatureO₂ release, which aids in the removal of O₂ bubbles during the meltingand fining stages of glass production results in a glass sheet that isessentially free of gaseous inclusions. Furthermore, any residual oxygenbubbles are reabsorbed by the fining agent due to transition from thereduced to oxidized state on cooling. However, from an environmentalpoint of view it is not desirable to use arsenic since it is considereda hazardous material.

There are several other known methods that do not need arsenic finingagents to mitigate oxidation reactions which lead to the formation ofgaseous inclusions in the glass sheets. One such method is described inU.S. Pat. No. 5,785,726 which discloses a humidity controlled enclosurethat surrounds one or more platinum-containing vessels and is used tocontrol the partial pressure of hydrogen outside the vessel(s) so as toreduce the formation of gaseous inclusions in glass sheets. Thishumidity controlled enclosure is discussed in more detail below.Although the method disclosed in the patent mentioned above successfullyreduces the formation of gaseous inclusions in the glass sheets, itwould be desirable to provide an alternative method to prevent theformation of gaseous inclusions in glass sheets. This need and otherneeds are satisfied by the system and method of the present invention.

SUMMARY

The present invention includes a system and method for controlling theenvironment (e.g., oxygen, hydrogen, humidity, temperature, gas flowrate) around one or more vessels in a glass manufacturing system. In thepreferred embodiment, the system includes a closed-loop control systemand a capsule that are used to control the level of hydrogen around theexterior (non glass contact surface) of the vessel(s) so as to suppressthe formation of gaseous inclusions and surface blisters in glasssheets. In addition, the closed-loop control system and capsule can beused to help cool molten glass while the molten glass travels from onevessel to another vessel in the glass manufacturing system. Moreover,the closed-loop control system and capsule can be used to maintain anatmosphere with minimal oxygen around the vessel(s) so as to reduce theoxidation of precious metals on the vessel(s).

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be had byreference to the following detailed description when taken inconjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram that shows the components of a glassmanufacturing system in accordance with the present invention;

FIG. 2 is a graph which shows the amount of blister generation (measuredin area coverage of blisters) versus hydrogen level in the atmosphere onthe exterior surface of a platinum glass processing vessel that could beused in the exemplary glass manufacturing system shown in FIG. 1;

FIG. 3 is a graph that is used to help explain the different operatingconditions in terms of ppm hydrogen vs. temperature that are possiblewith known techniques and with the invention;

FIG. 4 is a photograph showing two glass samples that were melted in anidentical platinum glass processing vessel for about ten minutes, one ofthe samples was processed in accordance with known techniques, the othersample was processed in accordance with the invention; and

FIG. 5 is a flowchart illustrating the basic steps of a method formaking a glass sheet in accordance with the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown a schematic view of an exemplaryglass manufacturing system 100 that uses the fusion process to makeglass sheets 137 in accordance with the present invention. The glassmanufacturing system 100 includes a melting vessel 110 in which batchmaterials are introduced as shown by arrow 112 and then melted to formmolten glass 114. The melting vessel 110 is typically made from arefractory material. The glass manufacturing system 100 further includescomponents that are typically made from platinum or platinum-containingmetals such as Pt—Rh, Pt—Ir, etc, and combinations thereof. Theplatinum-containing components include a premelt to finer connectiontube (PMFCT) 113, fining vessel 115 (e.g., finer tube 115), a mixingvessel 120 (e.g., stir chamber 120), a finer to stir chamber connectingtube 122, a delivery vessel 125 (e.g., bowl 125), a stir chamber to bowlconnecting tube 127, a downcomer 130 and an inlet 132. The inlet 132 iscoupled to a forming vessel 135 (e.g., fusion pipe 135) which forms theglass sheet 137. Typically, the forming vessel 135 is made from arefractory material.

In one embodiment of the present invention, the melting/delivery system141 which includes vessels 115, 120, 125 and tubes 122, 127 and 130 isencapsulated or encased within a capsule 140. A jacket volume 142 isdefined between the interior walls of the capsule 140 and the exteriorwalls of the components 115, 120, 122, 125, 127 and 130 in themelting/delivery system 141. The capsule 140 is preferably leak tight tothe extent that it may be used for maintaining a slightly more positivepressure of low oxygen, moist atmosphere inside the jacket volume 142that is greater than the ambient conditions. As shown, the capsule 140can be made as one zone which encloses the entire length of themelting/delivery system 141. Alternatively, multiple capsules 140 can beused as multiple zones where individual capsules 140 enclose one or moreof the vessels 115, 120, 125 and tubes 122, 127 and 130. An advantage ofutilizing multiple capsules 140 is the ability to independently controlthe atmosphere in a particular area of the melting/delivery system 141.

The present invention also includes a closed-loop control system 144which controls the environment/atmosphere within the capsule 140 andprevents the problematical oxidation reactions from occurring at themetal/glass interface inside components 115, 120, 122, 125, 127 and 130.Again, the problematical oxidation reactions lead to the formation ofgaseous inclusions in the glass sheet 137. In addition, problematicaloxidation reactions with the precious metal vessels and tubes can leadto the failure of the platinum (or other precious metals) on thecomponents 115, 120, 122, 125, 127 and 130.

In particular, the closed-loop control system 144 controls theatmosphere inside the capsule 140 so as to suppress undesirableoxidation reactions at the metal/glass interface by causing themigration of hydrogen into the glass/metal interface. A controlled levelof hydrogen permeation into the glass/metal interface reduces theproduction of undesirable species such as molecular oxygen, andhalogens, which in turn prevents the formation of undesirable gaseousinclusions in the molten glass 114. The hydrogen permeation into theglass/metal interface is achieved by supplying a higher partial pressureof hydrogen to the exterior surfaces (non glass contact surfaces) in themixing/delivery system 141, relative to the interior glass/metalinterfaces. To accomplish this, a humid, low oxygen atmosphere, whichresults in a controlled level of hydrogen at the non-glass contactsurface of the platinum system that is preferably greater than 12 ppm at1650° C., is maintained inside the capsule 140. It should be noted thatthe hydrogen level in the atmosphere inside the capsule 140 has anundetectable amount of hydrogen. However, hydrogen is generated when thewater breaks down at the high temperatures associated with the moltenglass 114. One gas system that could be used to create this atmospherewould be a mixture of water vapor, oxygen and nitrogen (or another inertgas like argon or helium). An exemplary closed-loop control system 144that uses this gas system to create such an atmosphere inside thecapsule 140 is described next.

The exemplary closed-loop control system 144 includes a controller 150that obtains sensor readings from one or more locations within andoutside the capsule 140. As shown, the controller 150 can obtain sensorreadings from capsule supply sensors 152, capsule sensors 154 andcapsule exit sensors 156 and 156′. In this example, the capsule supplysensors 152 include a flow sensor 152 a, a dew point/humidity sensor 152b, a temperature sensor 152 c, an oxygen sensor 152 d, and a pressuresensor 152 e. The capsule sensors 154 include a flow sensor 154 a, a dewpoint/humidity sensor 154 b, a temperature sensor 154 c, an oxygensensor 154 d, and a pressure sensor 154 e. And, the capsule exit sensors156 and 156′ each include a flow sensor 156 a and 156 a′, a dewpoint/humidity sensor 156 b and 156 b′, a temperature sensor 156 c and156 c′, an oxygen sensor 156 d and 156 d′, and a pressure sensor 156 eand 156 e′.

The controller 150 processes the sensor measurements and controlsdifferent devices like a humidity feed system 158, a heating/coolingcontrol system 160, air handler(s) 162 and an O₂/N₂ makeup system 164.The air handler(s) 162 have access to air and steam. All of the devices158, 160, 162 and 164 are connected to a network of pipes 166 which asshown is connected to the capsule 140. In operation, the controller 150controls the devices 158, 160, 162 and 164 to create anenvironment/atmosphere inside the capsule 140 where the hydrogen whichis generated by the decomposition of water vapor is done so at a ratethat is equal to or greater than the rate of hydrogen permeation throughthe metal walls of components 115, 120, 122, 125, 127 and 130 that wouldbe occurring if an ambient atmosphere were present at the non-glasscontact surface of the components. And, when there is a higher partialpressure of hydrogen, the reduction of undesirable species such asmolecular oxygen, and halogens within the molten glass 114 prevents theformation of undesirable gaseous inclusions in the molten glass 114.Another advantage of having a higher pressure of hydrogen is that therate of oxidation of the platinum containing components 115, 120, 122,125, 127 and 130 is reduced or possible eliminated due to the low levelof oxygen inside the capsule 140.

To suppress the formation of inclusions in molten glass 114, the levelof hydrogen on the exterior surfaces of the platinum containingcomponents 115, 120, 122, 125, 127 and 130 needs to be equal to orgreater than the level of hydrogen on the inside surfaces of thecomponents 115, 120, 122, 125, 127 and 130. The hydrogen level on theexterior surfaces of the platinum containing components 115, 120, 122,125, 127 and 130 is determined by the thermodynamic equilibrium of thewater decomposition reactionH₂O→H₂+½O₂.

In accordance with thermodynamic tables, the free energy (ΔG) for thewater decomposition reaction is equal to 58,900−13.1T, where T is thetemperature in degrees Kelvin and G is the free energy in calories permole. At a given temperature, the equilibrium constant for the waterreaction can be calculated by using the relationship K_(eq)=e^(−G/RT),where G and T are as previously noted, and R is the universal gasconstant. Once K_(eq) is known, the ratio of the partial pressures ofthe various gases involved in the water breakdown can be calculatedwhereK_(eq)=[(pH₂)(pO₂)^(1/2)]/pH₂O.

For example, at 1450° C., K_(eq) is equal to 2.47×10⁻⁵. Thus, if a 75°F. dew point air environment (pH₂O of 0.030 atmospheres) is heated to1450° C., then pH₂ is calculated to be 1.59×10⁻⁶ atmospheres (1.59 ppm).In view of this equilibrium, one can readily see that by lowering thepartial pressure of oxygen, while maintaining a constant dew point(pH₂O) one can substantially increase the hydrogen level in theatmosphere. It should be noted that the presence of nitrogen (or otherinert gas) in the preferred gas mixture does not directly participate inthe water decomposition reaction. Instead, the partial pressure of theinert gas affects the partial pressure of oxygen in accordance with theideal gas law. And, the change in partial pressure of oxygen is whatinfluences the equilibrium gases formed, due to the water decomposition.

Table 1 shows the effect of water and oxygen level on the level ofhydrogen at various temperatures in the enclosed environments of atraditional enclosure and the capsule 140.

TABLE 1 Traditional 1% 0.01% Capsule Enclosure Oxygen Oxygen 140 Dew Pt.(° F.) 80 80 80 140 O₂ (%) 21 1 0.01 0.5 pH₂ 1250° C. 0.2 0.9 9 8 (ppm)@ 1450° C. 2 9 88 77 1650° C. 11 52 524 463

The traditional enclosure is a room size enclosure that was made inaccordance with one embodiment of the invention in the aforementionedU.S. Pat. No. 5,785,726.

The traditional enclosure ensures that the partial pressure of hydrogenoutside components 115, 120, 122, 125, 127 and 130 in themelting/delivery system 141 is in an amount sufficient to preventformation of oxygen blisters in the glass that is adjacent to thevessel/glass interface. Although the traditional enclosure successfullyreduces the formation of gaseous inclusions in glass sheets it still hassome drawbacks. First, the traditional enclosure is so large that itmakes it difficult if not impossible to maintain a uniform environmentaround the components 115, 120, 122, 125, 127 and 130 in themelting/delivery system 141. Second, the traditional enclosure is solarge and the environment is so hot and humid that it can beuncomfortable to people that walk into the enclosure.

The capsule 140 and closed-loop control system 144 of the presentinvention addresses these drawbacks and other drawbacks associated withthe traditional enclosure. In the preferred embodiment, the capsule 140is a relatively small enclosure that produces a small jacket volume 142which facilitates better control of the atmosphere. This is due to factthat a probe reading (such as relative humidity or dew pointtemperature) for conditions inside the capsule 140 is more likely to berepresentative of conditions at the exterior metal surfaces of glassprocessing equipment because the volume in the capsule 140 is smallerthan the volume in the traditional enclosure. In addition, if there is aprocess instability or change in the water content of the molten glass114 that leads to an increase in hydrogen permeation blistering, thenthere is often no way to respond to this problem using the traditionalenclosure since it may be operating at its maximum dewpoint. The capsule140 and closed-loop control system 144 has a better chance of solvingthis problem.

As can be seen, the capsule 140 and closed-loop control system 144 ofthe present invention is essentially an enhanced version of thetraditional enclosure. Again, the traditional enclosure uses ahumidified air atmosphere around the metal portion of themelting/delivery system 141. And, the capsule 140 and closed-loopcontrol system 114 create a low oxygen moist atmosphere which allowshydrogen levels one to two or more orders of magnitude greater than thatwhich is possible with the use of a high dew point air atmosphere in thetraditional enclosure. The creation of this low oxygen moist atmospherealso extends the range of glasses that can be protected from hydrogenpermeation blistering.

Referring to FIG. 2, there is a graph which shows the amount of blistergeneration (area coverage of blisters on the glass contact surface ofplatinum) vs. hydrogen level in the atmosphere on the exterior (nonglass contact surface) of a platinum apparatus. As can be seen, the lowhydrogen levels commonly associated with the humid air atmosphere in thetraditional enclosure had unacceptable blistering over a wide range oftemperatures. And, the high hydrogen atmosphere associated with thecapsule 140 and the closed-loop control system 144 was very effective insuppressing the blistering in glass. Again, the traditional enclosureworks well but the capsule 140 and the closed-loop control system 144 ofthe present invention work even better at allowing blister suppressionover a broad range of temperatures and glasses.

Referring to TABLE 1 and FIG. 2, it can be seen that it is difficult forthe traditional enclosure to maintain an atmosphere with 12 ppm ofhydrogen at 1650° C. This is because with the traditional enclosure itis not possible to create a low oxygen atmosphere since people can andoften enter and exit the enclosed room. FIG. 3 is a graph thatillustrates this difference and other differences in terms of ppmhydrogen vs. temperature as they relate to different operatingconditions within the traditional enclosure and the capsule 140.Typically, the area above the curve 302 is the area where the capsule140 can operate but it would be difficult to operate the traditionalenclosure. And, both the capsule 140 and the traditional enclosure caneffectively operate in the area below the curve 302.

In view of FIG. 3, it can be seen that in order to improve upon thetraditional enclosure then the hydrogen level in the atmosphere of thecapsule 140 should be greater than or equal to the hydrogen levelcalculated in the following equation which uses the equilibriumrelationship:

$\begin{matrix}{{{pH}_{2}\mspace{11mu}({ppm})} = {78000 \times {\mathbb{e}}^{- \frac{G}{RT}}}} & (1)\end{matrix}$where G, R and T have been previously defined. This equation and thegraph shown in FIG. 3 were based on the pH₂O and pO₂ conditions in atraditional enclosure that tops out at a 80° F. dew point. In addition,this equation can re-written in numerical form as:

$\begin{matrix}{{{pH}_{2}\mspace{11mu}({ppm})} = {78000 \times {\mathbb{e}}^{- \frac{58900 + {13.1\; T}}{1.987\; T}}}} & (2)\end{matrix}$where temperature is in degrees Kelvin.

An example of the impact that a moist, low oxygen atmosphere has onglass is shown in FIG. 4. The photograph in FIG. 4 shows two glasssamples that were melted in identical platinum vessels that were 0.005″thick for ten minutes at 1450° C. The glass on the right was meltedusing known techniques in a 20° C. dew point air atmosphere, while theglass on the left was melted in accordance with the invention in a 20°C. dew point atmosphere containing 0.01% oxygen. To highlight thebubbles that were generated at the platinum-glass interface, aftertesting, the platinum was peeled from the glass and modeling clay waspressed into the bubble areas. It is clear that the glass exposed to thereduced oxygen atmosphere, which had a higher level of hydrogen, hadsignificantly less blisters than the glass tested in air.

As described above, the closed-loop control system 144 controls themoist, low oxygen atmosphere within the capsule 140 to inhibit thegeneration of gaseous inclusions in the glass sheet 137. In thepreferred embodiment, the closed-loop control system 144 accomplishesthis by controlling a gas system that has a mixture of water vapor,oxygen and nitrogen within the capsule 140. The typical values of oxygenwould be from 0.01% to 1%, water from 2 to 20%, with the balance of thegas being nitrogen (or another inert gas like argon). The gas systemcould be run as high as 21% oxygen and have a dew point as high as 200°F. And, a gas system with 0.01% oxygen and 20% water at a 200° F. dewpoint can give a range of hydrogen from 1 to 38,000 ppm at 1700° C.Alternatively, the mixture of gases introduced into the jacket volume142 of the capsule 140 may include hydrocarbons (and oxygen), ammonia,cracked ammonia products and/or combustion products.

Referring again to FIG. 1, the glass manufacturing system 100 can alsoincorporate two optional enhancements which are described next. Thefirst enhancement involves the use of a constriction plate 174 (orsimilar device) within the capsule 140 that constricts the flow of gasover a certain section or sections of the mixing/delivery system 141. Inthe preferred embodiment, the constriction plate 174 is located at anend of the fining vessel 115 such that 95% (for example) of the gas isdiverted into pipe 166 a and 5% (for example) of the gas flows over thefining vessel 155 and exits through pipe 166 b. This configurationenables one to design a laminar flow of gas within the capsule 140 whichenhances the ability to control of the environment. Alternatively, theconstriction plate 174 would not be needed if the capsule 141 was shapedsuch that less volume is present in a particular area between themixing/delivery system 141 over which less gas flow would be desired.

The second enhancement involves the use of pipe 166 c which provides oneway to cool one or more specific components 115, 120, 122, 125, 127 and130 in melting/delivery system 141. In this example, the finer to stirchamber connecting tube 122 (FSC tube 122) is cooled. As shown, pipe 166c has one end 168 a connected to the main pipe 166 at a location priorto entering the capsule 140. And, pipe 166 c has another end 168 b thatis directly connected to an inlet 170 that provides gas flow around theFSC tube 122. The FSC tube 122 also has an outlet 172 through which thegas mixture from pipe 166 c is directed back into the atmosphere withinthe capsule 140. The second enhancement is an important aspect of thepresent invention in that it helps one to better control the heattransfer in the glass manufacturing system 100. This heat transfercontrol can be done at the same time the present invention is used tocontrol the atmosphere for hydrogen permeation control.

In another aspect of the present invention, the capsule 140 and theclosed-loop control system 144 can be used to cool the components 115,120, 122, 125, 127 and 130 in the melting/delivery system 141 evenwithout the second enhancement. In particular, the present invention canbe used to help cool the molten glass 114 when it is moved from hightemperature conditions suitable for melting to lower temperatureconditions suitable for forming. Typically, the glass 114 needs to becooled about 400° C. To help cool the molten glass 114, the capsule 140and closed-loop control system 144 uses forced convection which isrelated to the gas flow outside the components 115, 120, 122, 125, 127and 130. Since, the capsule 140 is relatively small and has openings atknown locations which are used to connect to pipes 166, 166 a, 166 b and166 c, the heat transfer can be carefully controlled, and coolingperformance can be replicated from one installation to the next.Moreover, the laminar flow associated with the first enhancementdescribed above can also be used to help one better control the heattransfer.

In contrast, the traditional enclosure has difficulty controlling theheat transfer because the gas flow is uncontrolled and depends on thedifferent local temperatures and air flows within the enclosed room. Asa result, the local cooling rate on the mixing/delivery system 141 inthe traditional enclosure can only be controlled by the raising orlowering of local zone electrical heating devices. However, the localcooling rate can be adjusted inside the capsule 140 by both heater powerand gas flow rate. This allows a larger range of cooling controlcapacity for glass flow as indicated in TABLE 2.

TABLE 2 Glass Flow Capsule 140 Traditional Enclosure Nominal X X Minimum0.75X  0.85X Maximum 1.35X 1.2X

As shown in TABLE 2, the traditional enclosure has a minimum glass flowboundary of 0.85× which occurs when the heater power reaches itsmaximum. And, the traditional enclosure has a maximum glass flowboundary of 1.2× which occurs when the heater power is turned back farenough that some circuits are turned off. If the circuits are turnedoff, then the effective control of cooling is lost. The capsule 140enables these boundaries to be extended to 0.75×-1.35× (for example)when air cooling is also variable. In addition, the flow boundariesassociated with the capsule 140 are dictated by cooling capacity, not byhead loss or other considerations. Thus, the capsule 140 enables one tocontrol the cooling by forced convection. This type of control is notpossible in the traditional enclosure because the enclosed room is solarge and personnel can enter and leave the enclosed room.

Referring to FIG. 5, there is a flowchart illustrating the basic stepsof a method 500 for making a glass sheet 137 in accordance with thepresent invention. Beginning at step 502, molten glass 114 is formed andprocessed within a melting vessel 110, a fining vessel 115, a mixingvessel 120, a delivery vessel 125, and a forming vessel 135. At step504, the capsule 140 is used to enclose one or more of these vessels110, 115, 120, 125 and 135 and tubes 113, 122, 127 and 130. In thepreferred embodiment, the capsule 140 encloses the vessels 110, 115, 120and 125 and tubes 122, 127 and 130 which are associated with themelting/delivery system 141 (see FIG. 1). At step 506, the closed-loopcontrol system 144 is used to create, monitor and control a humid, lowoxygen atmosphere within the capsule 140. In the preferred embodiment,the closed-loop control system 144 is used to control the level ofhydrogen within a mixture of gases so that at least 12 ppm of hydrogenat 1650° C. (for example)(see TABLE 1, and FIGS. 2-3) is present aroundthe exteriors of the enclosed vessels 110, 115, 120 and 125 so as toreduce hydrogen permeation from the molten glass 114 and effectivelysuppress the formation of undesirable gaseous inclusions within themolten glass 114. In addition, the closed-loop control system 144 can beused to control the level of oxygen that is present around the exteriorsof the enclosed vessels 110, 115, 120 and 125 so as to reduce theoxidation of the precious metal on the enclosed vessels 110, 115, 120and 125 and tubes 122, 127 and 130. Moreover, the closed-loop controlsystem 144 can be used to control the cooling of the molten glass 114while it travels from one of the vessels (fining vessel 115) to anotherone of the vessels (mixing vessel 120).

Following are some advantages, features and uses of the presentinvention:

The present invention could be used by any glass manufacturer thatmelts, delivers or forms glass in a system in which the glass contacts aprecious metal device having one surface in contact with the glass andthe other surface being a non-contact glass surface. This precious metaldevice does not need to be a vessel but instead could be some otherdevice like a thermocouple sheath, stirrer or bowl liner (for example).In addition, the present invention could be beneficial in themanufacturing of Vycor tubing and sheet. Moreover, the present inventionis beneficial in the manufacturing any type of glass product.

The present invention reduces the oxidation of the external surfaces ofthe platinum containing components. Current technology relies on acoating, such as Rokide (aluminum oxide) that is placed on the outersurface of platinum containing components to limit the contact of air(oxygen) with the precious metal. This invention provides a means oflowering the oxygen level, which is a key driver in the undesirableoxidation reaction of platinum. There are many advantages to using aninert or reducing atmosphere to prevent platinum oxidation. First, theremoval/reduction of oxygen decreases the rate of oxidation by orders ofmagnitude. The best coatings typically decrease oxidation by a factor of2 to 4×. Secondly, the removal/reduction of oxygen eliminates the needto use thicker sections of platinum in the vessels to stop failures dueto oxidation. As a result, the cost for vessels would be less than asystem designed with thicker sections for improved life. Thirdly, theuse of an inert or reduced gas coverage within the capsule 140 makes itpossible to protect all areas of the precious metal system, even theareas that are intricate in shape which would be difficult to coat.

The present invention can be used in any glass or melting system inwhich glass comes in contact with precious metals such as gold,platinum, rhodium, iridium, molybdenum, palladium, rhenium tantalum,titanium, tungsten and alloys thereof. This contact could be in themelting, delivery or forming phase of production.

This present invention eliminates the need for adding multivalentspecies (fining agents) in the glass such as arsenic and antimony oxidesto buffer oxidation reactions at the Platinum glass interface. And, if amultivalent specie is needed for fining, its concentration can beminimized. In addition, multivalent species that are less effective asfining agents can if needed be used, that are not considered hazardousmaterials. This increases the number of possible glass compositions andalso allows for a fully environmentally friendly glass to be produced.

The present invention requires no internal intervention into the glassmelting/delivery system 141, and can be applied anywhere on the systemfrom the external surface.

The capsule 140 can be a simple container or barrier that is capable ofmaintaining a positive pressure of the low oxygen environment. Forinstance, the capsule 140 can be something as simple as a plastic orrubber bag or something that is more permanent like an enclosure whichis shown in FIG. 1.

It should be appreciated that the capsule 140 can also enclose othercomponents in the mixing/delivery system 141 in addition to components115, 120, 125 and tubes 122, 127 and 130. For instance, the capsule 140can also enclose components 113 and 132.

It should also be appreciated that the capsule 140 can have more or lessinlets and outlets than shown in FIG. 1.

A user of the present invention does not have to be concerned abouthaving too high a level of hydrogen around the melting/delivery system141 which can impact the integrity of the precious metal vessels.Because, the present invention uses a nitrogen, water, oxygenenvironment which makes it difficult if not impossible to get hydrogenlevels to the extent that glass constituents (e.g., Fe, Sn, As, Sb)would be reduced, causing the metal of the system to be attacked anddestroyed.

For more details about the aforementioned fusion process reference ismade to U.S. Pat. Nos. 3,338,696 and 3,682,609. The contents of thesetwo patents are incorporated herein by reference.

Although one embodiment of the present invention has been illustrated inthe accompanying Drawings and described in the foregoing DetailedDescription, it should be understood that the invention is not limitedto the embodiment disclosed, but is capable of numerous rearrangements,modifications and substitutions without departing from the spirit of theinvention as set forth and defined by the following claims.

1. A method for producing a glass product, said method comprising thesteps of: forming and/or processing molten glass within at least one ofa melting, a fining, a delivery, and a forming vessel; providing acapsule to enclose at least one of the vessels; and providing anatmosphere between interior walls of the capsule and around a non-glasscontact surface of said at least one vessel, wherein said step ofproviding an atmosphere further includes: controlling a level ofhydrogen within a mixture of gases in the atmosphere so that a partialpressure around the non-glass contact surface of said at least onevessel is maintained at or greater than a level defined by anequilibrium relationship: $\begin{matrix}{{{pH}_{2}\mspace{11mu}({ppm})} = {78000 \times {\mathbb{e}}^{- \frac{58900 + {13.1\; T}}{1.987\; T}}}} & (2)\end{matrix}$ to suppress the formation of undesirable gaseousinclusions within the molten glass, where T is a temperature in Kelvinof the non-glass contact surface of said at least one vessel.
 2. Themethod of claim 1, wherein said step of providing an atmosphere furtherincludes the step of controlling a level of oxygen within the gasmixture around the non-glass contact surface of said at least one vesselto reduce oxidation of a precious metal on said at least one vessel. 3.The method of claim 1, wherein said step of providing an atmospherefurther includes the step of controlling a cooling of the molten glasswhile the molten glass travels from one of said at least one vessel toanother one of said at least one vessel.
 4. The method of claim 1,wherein said gas mixture results in up to 38,000 ppm of hydrogen at1700° C. at the non-glass contact surface of said at least one vessel.5. The method of claim 1, wherein said gas mixture is maintained at adew point temperature of 200° F. or lower.
 6. The method of claim 1,wherein said gas mixture has an oxygen content with a level of less than21% by volume.
 7. The method of claim 1, wherein said gas mixture has anoxygen level of 0.01% to 1% by volume and a water vapor level of 2% to20% by volume, with the balance being essentially an inert gas.
 8. Themethod of claim 1, wherein said gas mixture includes cracked ammoniaproducts.
 9. The method of claim 1, wherein said non-glass contactsurface of said at least one vessel includes a metal selected from gold,platinum, rhodium, iridium, molybdenum, palladium, rhenium, tantalum,titanium, tungsten and alloys thereof.
 10. The method of claim 1,further comprising a step of positioning a constriction plate around thenon-glass contact surface of one of said at least one vessel thatreduces a flow of the gas mixture over that vessel and causes a laminarflow of the gas mixture within said capsule.
 11. The method of claim 1,wherein controlling a level of hydrogen within the atmosphere includesusing a closed-loop control system to monitor and control thecomposition of the atmosphere.
 12. The method of claim 11, where saidclosed-loop control system includes: a controller; a plurality ofsensors; a humidity feed system; a heating/cooling control system; anair handler; and an O₂/N₂ makeup system.
 13. The method of claim 12,wherein the humidity feed system, the heating/cooling control system,the air handler, and the O₂/N₂ makeup system are connected to a networkof pipes which are connected to the capsule.
 14. The method of claim 13,wherein the network of pipes includes a main pipe attached to a firstend of the capsule and the network of pipes further includes an exitpipe attached to an opposite end of the capsule.
 15. The method of claim14, wherein the network of pipes further includes another pipe which hasone end attached to the main pipe and another end connected to an inletof a tube used to connect a pair of the at least one vessels where thegas mixture flows around the tube which also has an outlet through whichthe gas mixture is directed back into an atmosphere within the capsule.16. The method of claim 12, wherein said controller processes sensormeasurements from the plurality of sensors and controls the humidityfeed system, the heating/cooling control system, the air handler, andthe O₂/N₂ makeup system.
 17. The method of claim 12, wherein saidcontroller obtains sensor readings from the plurality of sensorsincluding capsule supply sensors, capsule sensors and capsule exitsensors.
 18. The method of claim 17, wherein said capsule supply sensorsinclude a flow sensor, a dew point/humidity sensor, a temperaturesensor, an oxygen sensor, and a pressure sensor.
 19. The method of claim17, wherein said capsule sensors include a flow sensor, a dewpoint/humidity sensor, a temperature sensor, an oxygen sensor, and apressure sensor.
 20. The method of claim 17, wherein said capsule exitsensors include a flow sensor, a dew point/humidity sensor, atemperature sensor, an oxygen sensor, and a pressure sensor.
 21. Themethod of claim 3, wherein said step of controlling the cooling of themolten glass further includes using forced convection to cool the moltenglass while the molten glass travels from one of said at least onevessel to another one of said at least one vessel.
 22. The method ofclaim 1, wherein the vessel enclosed by the capsule is permeable tohydrogen gas at the operating temperature thereof.
 23. The method ofclaim 1, wherein the step of providing an atmosphere includescontrolling the partial pressure of oxygen in the atmosphere to below0.21 atmosphere.
 24. The method of claim 1, wherein the step ofproviding an atmosphere includes providing an atmosphere having asubstantially constant composition and temperature distribution duringoperation of the vessel.
 25. The method of claim 1, wherein the capsuleand the non glass-contact surface of the vessel enclosed by the capsuledefines a space to which human access is not needed during normaloperation of the capsule.